Optical pulses are essential ingredients in the deployment of imaging systems, precision manufacturing and high-speed communications. The ability to manipulate their properties, including their spectral bandwidth is therefore an important function.
However, most spectral tuning mechanisms today rely on bandpass filtering, which inadvertently leads to losses in pulse energy. Specific examples of such approaches include the use of acousto-optic filters, fiber Bragg gratings and resonant filters, all of which require truncating the pulse spectrum to achieve spectrally narrower and temporally longer pulses. Approaches which allow spectral tuning of optical pulses without pulse energy losses are therefore highly sought after.
The research group led by Prof. Dawn T.H. Tan from Singapore University of Technology and Design developed a new approach which has been demonstrated for tunable pulse spectra without losses in pulse energy typically associated with spectral truncation. This work was published in Photonics Research, Vol. 9, No. 4, 2021(Yanmei Cao, Ezgi Sahin, Ju Won Choi, Peng Xing, George F. R. Chen, D. K. T. Ng, Benjamin J. Eggleton, Dawn T. H. Tan. Thermo-optically tunable spectral broadening in a nonlinear ultra-silicon-rich nitride Bragg grating[J]. Photonics Research, 2021, 9(4): 04000596).
The method combines the Kerr nonlinearity and the thermo-optic effect present in a complementary metal-oxide semiconductor compatible photonic device to achieve the energy efficient spectral tuning function. Based on the ultra-silicon-rich nitride (USRN) platform which possesses a nonlinear refractive index, 100X larger than in stoichiometric silicon nitride, the spectral tuning device consists of a two-stage device: a cladding-modulated Bragg grating concatenated with a highly nonlinear waveguide.
In the first stage, pulses propagating in the cladding-modulated Bragg grating experience strong grating induced dispersion, three orders of magnitude larger than in a waveguide, with the magnitude decreasing away from the stopband.
When situated on the blue side of the grating, these pulses evolve into Bragg solitons, undergoing high-order temporal compression. The extent of compression varies with the detuning between the grating stopband and the pulse wavelength. Stronger compression results in pulses which then undergo more pronounced spectral broadening in the second stage of the device, through the process of self-phase modulation.
Consequently, varying the extent of Bragg soliton effect compression in the first stage can be achieved by tuning the detuning between the pulse wavelength and the grating stopband. This requirement brings us to the second essential ingredient for spectrally tunable pulses. For a fixed pulse wavelength, detuning between the pulse wavelength and the grating stopband can only be achieved by leveraging mechanisms to shift the location of the grating stopband.
Serendipitously, USRN possesses a large thermo-optic coefficient, an order of magnitude larger than that in stoichiometric silicon nitride. Consequently, increasing the temperature from 25oC to 70oC allows the location of the grating stopband to increase by 5 nm. As a result, the detuning between the pulse wavelength and the grating stopband increases. This in turn results in a continuous decrease in the dispersion experienced by the pulse, a reduction in the temporal compression and therefore a smaller extent of spectral broadening in the USRN waveguide.
Thermo-optic tuning leveraging Bragg soliton dynamics allows a wide dynamic range of spectral widths to be continuously accessed without loss in pulse energy. The combination of a strong Kerr nonlinearity and large thermo-optic coefficient in ultra-silicon-rich nitride allows energy efficient, thermo-optic spectral tuning of laser pulses to be achieved at low powers, as shown in Fig.1.
Fig.1 Simulated spectral broadening at different temperatures at a fixed pulse wavelength of 1536 nm.
Experiments to confirm this effect show that a variation on the 30 dB bandwidth from 69 nm to 106 nm is possible as the temperature of the device is increased from 25oC to 70oC.
Importantly, the technique is shown to be significantly more energy efficient than other spectral tuning methods. The wide dynamic range of spectral tuning, particularly with the high energy efficiency are important with considering deployment in practical devices.
From a fundamental standpoint, this work's significance further stems its application of a relatively new concept of on-chip Bragg solitons. While previously studied in fiber, Bragg solitons have only recently been demonstrated on a chip. Combining the augmented dispersion with the large nonlinearities available in high nonlinear figure of merit USRN waveguides, Bragg soliton effects can easily be observed at substantially lower powers than that in fibers. Indeed, much more remains to be studied, and applications discovered, in this exciting class of on-chip devices.
"This technique is especially well suited to integration with short pulse lasers, providing a practical avenue for an easily implemented, low loss technique for the tuning of pulse spectra," says Cao Yanmei, the PhD student and first author of the work.
"The tuning of pulse spectra in turn allows the temporal duration of pulses to be likewise tuned, a feature which is most useful for short pulse laser applications within the domains of imaging, precision manufacturing and high-speed communications," explains Prof. Dawn Tan who supervised this work.
The technique can be advanced further by increasing the dynamic range of the spectral tunability. In addition, mechanisms other than the thermo-optic effect which allow the stopband of the grating to be tuned could also be investigated, perhaps allowing new paradigms of ultrafast spectral tuning of pulses to be discovered.
